In a conventional closed loop fuel control system, a gas sensor is utilized to detect the concentration of a component of the exhaust gas where the gas sensor indicates the instantaneous air/fuel ratio of the air and fuel mixture supplied to the combustion engine. At least one electromagnetic valve disposed in an air-bleed of a carburetor controls the opening of the air-bleed in response to a control signal obtained by using a signal from the gas sensor and thus controls the air/fuel ratio. Also in an injection system, such a feed back system in which the fuel flow rate is controlled is proposed.
This closed loop fuel control system is mainly used for obtaining the maximum efficiency of a three-way catalytic converter interposed in the exhaust passage by maintaining a desired air/fuel ratio (as an example, in the vicinity of the stoichiometric air/fuel ratio). However, the carburetor system has a tendency to have a deviation between the actual characteristics of the fuel flow rate and the required one as shown in FIG. 1. This deviation is so large that it can not be ignored and the amount of the deviation varies in accordance with the condition of the engine operation.
In order to compensate this deviation the control range of the conventional closed loop fuel control system had to be designed very wide corresponding to such a large error. The above-mentioned control range may be represented by an ON/OFF rate of a valve provided to selectively open an air-bleed of a carburettor. FIG. 2 shows a typical valve ON/OFF rate of a conventional closed loop fuel control system at various engine operational conditions. As shown in this figure, the valve ON/OFF rate, i.e. the control range, widely varies in accordance with the engine operational conditions. When the engine operational conditions are changed, for example from an idling state to an accelerating state, the valve ON/OFF rate is suddenly changed from a very low rate to a high rate.
When the control range of the correction is made large for providing an adequate feed back control by using an ON-OFF type of an electromagnetic valve as described, the difference between the richest and the leanest air/fuel ratio becomes considerably large and this large difference may cause a hunting phenomenon of the internal combustion engine.
As shown in FIG. 2, time delays occur at the transient state between two different engine operational conditions. The delay times are denoted by .DELTA.t.sub.1, .DELTA.t.sub.2, .DELTA.t.sub.3 in the figure. These delay times are undesirable for obtaining high responsive characteristics.